Visa has pledged to compensate customers affected by its recent IT system failure after revealing 5.2 million card transactions were impacted, including 2.4 million in the UK.

The group said it was working with issuing banks to assess "appropriate compensation" for customers after the 10-hour debacle, which saw 9% of all Visa's UK transactions fail to go through, impacting millions of customers on a busy Friday afternoon.

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In a letter to Treasury Select Committee chair Nicky Morgan, Visa laid bare the extent of the issue, revealing that more than a third of all UK transactions failed to process on the first attempt at the height of the disruption - right throughout rush hour - forcing many people to use cash machines.

Visa Europe chief executive Charlotte Hogg said: "At its peak, the disruption affected people in the midst of returning home from work, socialising in restaurants and pubs and doing end-of-day shopping.

"A disruption to our processing that impacts consumers at any time is unacceptable, let alone during a busy Friday afternoon."

Her letter to the cross-party group of MPs comes after Mrs Morgan demanded details on the June 1 debacle and the "chaos" it caused.

In her response, Ms Hogg - a former Bank of England interest rate setter - said: "We failed to meet your and our own expectations and we apologise again unreservedly to everyone affected by the incident.

"Visa, together with our financial institution partners, has quickly implemented a compensation programme for cardholders in response."

Visa said the issue was caused by a "very rare partial failure" of a switch in one of its data centres, which meant its back-up centre could not automatically process all transactions.

It has since fixed the issue and said it was taking "all necessary steps" to prevent the failure happening again, while also improving its incident response.

As well as compensating customers where necessary, Visa said it was waiving fees for merchants on transactions that failed to process and was working on a programme to mitigate any processing related fees incurred.

But it insisted that cardholders will not have been charged for transactions that did not complete.

Visa has launched an internal review and has commissioned EY to conduct an independent inquiry into the incident and its response.

Mrs Morgan said the Treasury Committee was "satisfied with Visa's answers", seemingly sparing Ms Hogg another grilling in front of MPs.

But Mrs Morgan warned: "The detriment caused to consumers by IT failures is greater than ever, so the Committee will become less tolerant of them.

"The Committee expects to see the findings of the independent review, which will examine the lessons to be learned from the incident, in full."

The embarrassment comes just over a year since Ms Hogg was forced to resign from her role as deputy governor at the Bank after a previous ill-fated appearance in front of the Treasury Committee.

She had come under fire for breaking the Bank's code of conduct by failing to declare that her brother worked for Barclays.